The present invention relates to a self-illumination scanner apparatus which reads an image by illuminating a surface to be sensed including various kinds of optically readable images such as code patterns, characters, patterns, and the like printed on, e.g., a paper sheet using an illumination light-emitting member comprising an LED (light-emitting diode), and receiving light. reflected by the illuminated surface to be sensed.
Conventionally, various self-illumination scanner apparatuses of this type are known.
The assignee of the present invention has proposed a system for recording audio information on a recording medium such as a paper sheet in the form of a dot code which serves as image information, i.e., encoded information, that can be transmitted via a facsimile apparatus and can produce a large quantity of copies at low cost, and which is defined by a two-dimensional matrix of a plurality of dots, and also a system for reproducing the dot code, as disclosed in, e.g., EP0670555A1, which describes a self-illumination scanner apparatus for optically reading the dot code.
As an illumination light-emitting member for such self-illumination scanner apparatus, an LED is normally used since it is relatively inexpensive, is easily available, and can be driven at a lower voltage than other illumination light-emitting members.
However, LEDs have shortcomings, i.e., variations in internal resistance upon operation in units of elements.
For example, when a circuit arrangement shown in FIG. 1, i.e., a parallel circuit of a plurality of LEDs is used, the drive currents differ in units of elements at a ratio that depends on their internal resistance ratio, resulting in variations in the amount of light emitted by the respective LED elements.
As another shortcoming, since LEDs have variations in current-light amount conversion efficiency in units of elements, variations in drive current further promote variations in the amount of light emitted.
Consequently, when the circuit arrangement shown in FIG. 1 is used as an illumination light-emitting member in the above-mentioned self-illumination scanner apparatus, even if the plurality of LEDs are disposed at equal angular intervals in an annular pattern around a solid-image image sensing element, the brightness on the surface to be sensed becomes nonuniform due to. variations in light amount emitted by the respective LED elements, and each dot in a dot code is hard to accurately recognize and read as a dot.
In order to solve such problem, a circuit arrangement as a series circuit of a plurality of LEDs shown in FIG. 2 may be used.
According to this circuit arrangement, since the variations in internal resistance in units of LED elements are averaged by the serially connected LEDs, these variations can be suppressed consequently.
However, when the arrangement show in FIG. 2 is used, a high voltage must be applied to a terminal A, and a high-voltage generation circuit must be added.
More specifically, in the arrangement of EP0670555A1, since a plurality of LEDs are controlled to emit light instantaneously, the drive current required for obtaining a necessary light amount becomes high.
The anode-cathode voltage of each LED nonlinearly increases with increasing current. For this reason, when three LEDs are connected in series with each other, as shown in, e.g., FIG. 2, if a current of nearly 100 mA is applied to one column, the voltage across the terminals A and B becomes as high as about 7V due to the forward-bias voltage of the LEDs.
In practice, a drive circuit for the LEDs has an arrangement shown in FIG. 3.
In this case, since a constant current circuit alone requires several V, a high voltage of at least 10V is required at the terminal A.
Also, when a solid-state image sensing element such as a charge coupled device (CCD) or the like exclusively used in the self-illumination scanner apparatus is driven, a high voltage is required since a capacitive load must be driven.
Hence, the power supply for the illumination LEDs may be commonly used as that for driving this solid-state image sensing element.
Unfortunately, since the power supply for the LEDs has a large sag owing to heavy instantaneous load upon light emission, direct use of the power supply for the solid-state image sensing element that requires a stable power supply as that for the LEDs is extremely unpreferable.
In view of the foregoing, in order to realize the circuit arrangement shown in FIG. 2, an additional high-voltage generation circuit is required. However, if this circuit is merely added, another problem, i.e., an increase in size or weight of the apparatus, is posed.